The present invention relates to an image recording apparatus such as a copying machine or a printer, and more particularly, to an improvement on a thermal fixing unit or a heating unit for heating a developer sheet so as to provide glossy output image without any local image drop out.
In one conventional image recording apparatus, a recording medium is of transfer type recording medium such as a photosensitive sheet which comprises a microcapsule sheet (first image recording medium) and a developer sheet (second image recording medium). More specifically, in the transfer type recording mediums, the developer material is coated on a separate substrate as a separate developer or copy sheet. The first image recording medium comprises a photosensitive pressure sensitive recording medium provided with microcapsules which encapsulate therein a first material (chromogenic material or dye precursor) having first and second phases dependent on light exposure. The second image recording medium comprises the developer medium provided with a second material (developer material) which provides an output image upon reaction with the first material. The developer sheet includes a base substrate, the developer material layer and a surface resin layer. Such image recording medium is disclosed in, for example, U.S. Pat. No. 4,399,209 and Japanese Patent Application Kokai No. 58-88739.
As is well known, the image recording apparatus of this type irradiates light onto an original and a light reflected from the original or transmitted therethrough passes through an optical system such as a reflection mirror, a filter and lenses, and is focused on the photosensitive recording medium at an exposure unit. As a result, a latent image is formed on the photosensitive recording medium upon photocuring the light exposed microcapsules. Then, the light-exposed photosensitive recording medium is superposed with a developer sheet, and these sheets are pressed together at a pressure developing unit, so that non photo-cured microcapsules are ruptured to react the chromogenic material with the developer material, to thereby provide an output image on the developer sheet. After the pressure developing operation, the developer sheet is delivered to a thermal fixing unit where the output image is thermally fixed and glossy output image is obtained. Further, in case the developer sheet has a PET substrate, such sheet is used as an overhead projector sheet (OHP sheet). In this case, high transparency is required at a surface resin layer and the developer layer, and therefore, complete heating is required for melting these layers.
In the thermal fixing process, a heat roller is used which is in direct contact with the imaging surface of the developer sheet so as to positively heat the surface resin layer. However, upon heating the surface resin layer, viscosity of the resin is increased, and therefore, the output image may be transferred onto the heat roller surface. As a result, partial image drop out may occur at the resultant output image.
Further, in a conventional image recording apparatus, exposure, pressure development and thermal fixing operations are carried out irrespective of the characteristics of the sheet such as the developer sheet size and material of the developer sheet. Accordingly, resultant output image may be insufficient in terms of transparency, resolution power and Haze percent (Haze percent is defined in ASTM D1003).